1. Field of the Invention
With reference to the classifications of art established in the United States Patent Office, the substance of the present invention pertains to the general class "Hydraulic and Earth Engineering" (Class 61) and in particular to subclass "Piles" thereunder (Sub 53) and other subclasses as they may incorporate the materials once in and otherwise appropriate to now-discontinued Subclass 77.
2. Description of the Prior Art
In the operation of driving piles a so-called cushion is commonly used for prevention of physical damage to the hammer and/or the pile and pile follower apparatus caused by the repetitive, high impact induced stresses resulting from substantially instantaneous deceleration of the massive hammer ram from its high initial velocity. Under certain conditions secondary shock loads may occur due to pile rebound against the hammer. Other purposes of a cushion are to correct for slight misalignment between hammer and pile axes and to help assure full area contact between the striking surfaces.
It is rather common practice to use wood blocks or chips contained in a metal cylinder or helmet as a cushion means. However, wood in either block or chip form absorbs and retains a relatively large portion of the delivered energy and quickly becomes compacted; burns, and often must be replaced or replenished numerous times during the course of an operating day at the cost of operating continuity. In addition, the quantities of energy non-productively extracted or absorbed by wood cushions vary unpredictably during the course of hammering operations, and thus the allowances made or to be made for them are uncertain. This can result in inaccurate determinations of the pile bearing capacity when such determinations are calculated by assuming the hammer energy transferred to the pile through the cushion to be a known, fixed quantity.
Other forms of cushioning means are found in practice such as multilayered stacks of metal discs, stacks of alternating discs of aluminum and micarta, coils of metal cable, and other materials. By and large almost all of the known cushioning means actually used have been developed through trial and error methods including using practically any compressible materials at hand on a construction site. Although they may be workably adequate they are less than fully satisfactory.
Among the several patents showing cushion apparatus in the general area of the present invention are U.S. Pat. No. 1,943,420 to R. D. Budd, U.S. Pat. No. 2,184,745 to W. P. Kinneman, U.S. Pat. No. 2,723,532 to E. A. Smith, and U.S. Pat. No. 3,489,229 to P. Uebel et al. In the art disclosed by those patents one or more flat springs as hereinafter described and defined in the particular form of disc springs are used to absorb hammer energy. However, neither methods nor means for limiting disc deflection to minimize fatigue effects nor for containing the springs to avoid rebound, seal out dirt and harmful contaminants, promote lubrication and retain lubricants are taught. To be practical and efficient under field conditions a cushion utilizing spring elements is desirably self-contained or at least not comprised altogether of loose components; not susceptible to rebounding and possibly falling back out of working alignment; capable of giving reasonably long service and withstanding exposure to many kinds of adverse environmental influences or effects such as sand, dust, rain and snow, and needful of little or no attention. Also, means to indicate spring failure is highly desirable.